1. Field of the Invention
The present invention relates generally to magnetic heads that are utilized with thin film hard disk data storage devices, and more particularly to the design and fabrication of a magnetic head having an optical energy resonant cavity storage media heating device formed therein.
2. Description of the Prior Art
Hard disk drives generally include one or more rotatable data storage disks having a magnetic data storage layer formed thereon. Data in the form of small magnetized areas, termed magnetic data bits, are written onto the magnetic layers of the disks by a magnetic head that includes magnetic poles through which magnetic flux is caused to flow. Magnetic flux flowing from a pole tip portion of the magnetic poles in close proximity to the magnetic layer on the disk, causes the formation of the magnetic bits within the magnetic layer.
The continual quest for higher data recording densities of the magnetic media demands smaller magnetic data bit cells, in which the volume of recording material (grains) in the cells is decreased and/or the coercivity (Hc) is increased. When the bit cell size is sufficiently reduced, the problem of the superparamagnetic limit will provide a physical limit of the magnetic recording areal density. Present methods to delay the onset of this limit in storage media include the use of higher magnetic moment materials, and using thermally assisted recording heads. The present invention relates to such thermally assisted recording heads in which a heating device is disposed within the magnetic head. Heat from the heating device temporarily reduces the localized coercivity of the magnetic media, such that the magnetic head is able to record data bits within the media. Once the disk returns to ambient temperature, the very high coercivity of the magnetic media provides the bit stability necessary for the recorded data disk.
Thermally assisted recording (TAR) is potentially a powerful technique in advancing magnetic recording to the 1 Tbit/in2 range and beyond, and a promising technique includes the use of optical energy from the magnetic head to heat the media as it passes beneath the head. However, in using optical energy for the heating of the magnetic medium, one needs to consider the applicability of the optics in near field, e.g., 1 to 20 nm from the source which resides in the magnetic head slider, and the heating of an area in the medium of very small dimensions, e.g., in the 20 to 30 nm range. Conventional diffraction limited optics is not applicable for such a small area. Recently, descriptions of several TAR methods for near-field heating of media have been published. In published U.S. patent applications US2003/0184903 A1 and US2004/0008591 A1 special ridged waveguides used as high transmission apertures disposed within the magnetic head are taught for applications in perpendicular recording. In one example, a ridged waveguide is located immediately downtrack of the write pole such that the input plane of the ridged waveguide is parallel to the air bearing surface (ABS). In published U.S. patent applications US2004/0001420 A1 and US2004/0062503 A1 a planar waveguide is constructed on the downtrack side of the write pole. In this respect the heated spot is displaced downtrack of the write pole by the thickness of a cladding of the waveguide. In general the size of the heated spot depends on the optical wavelength and the dimensions and the composition of the materials for the waveguide/ridged waveguide.
In order to understand the operation of an optical cavity resonator it is useful to first consider the resonance of a simple circular cylindrical cavity at microwave frequencies, i.e., the cavity is a hollow circular cylinder. In microwave electronics, a closed circular cylinder has well defined resonances represented by transverse magnetic TMmnp modes and transverse electric TEmnp modes. The indices m, n, and p refer to the number of modes in the azimuthal, radial and longitudinal directions, respectively. For the present case, we limit our discussions to the fundamental mode, TM010 i.e., there are no variations in the azimuthal and longitudinal directions. In this simple mode the magnetic field for TM010 is concentric with the cylinder. On the other hand, the electric field is in the axial direction and has a maximum in the center of the cylinder. All its electric field lines span between the two side walls.
In an effort to increase the electric field in a circular cylindrical cavity, the reentrant cylindrical cavity resonator 12 shown in FIGS. 1A and 1B was developed for the generation of microwave power with klystrons and magnetrons. This cavity 12 is simply a circular cylindrical cavity with a coaxial post 16 which is shorter than the thickness of the cavity. The post extends from one side wall 20 and ends at a subwavelength distance, d, from the opposite side wall 24. An aperture 28, usually of subwavelength diameter, is placed in the side wall 24 opposite to the end of the post 16. The presence of the post 16 compounds the designation of the resonant modes. However, the cavity 12 is normally operated to resonate in a fundamental mode such that the electric and magnetic fields are axisymetric. Further, the electric field at the post remains parallel to the axis and peaks at or near the axis. Also, the magnetic field is perpendicular to the electric field such that its field lines are concentric circles about the axis. The presence of the post 16 intensifies the electric field at the axis because the post to side wall separation, d, is now smaller than the thickness of the cylinder. Generally, the smaller the magnitude of d, the greater the axial electric field strength along the axis. What these experiments in the microwave regime have demonstrated is that a reentrant cylindrical cavity of subwavelength dimensions can produce very high intensity electric field in a direction normal to the face 24 of the cavity, a fact that is extended in the present invention for near field thermal heating at optical wavelengths.
Since any modification to a resonant cavity, such as a post or an aperture, perturbs the simple TE and TM modes in the cavity, in the following we will refer to axial modes where the fields in the original, unmodified resonant cavity would be TM and will use in-plane to refer to fields which would have been TE in the unmodified resonant cavity. Thus for a circular cylindrical cavity, axial fields will imply that the electric field is predominantly oriented parallel to the axis of the cylinder and in-plane fields will imply that the electric field is predominantly perpendicular to the axis of the cylinder and thus in the plane of the air bearing surface.
An important consideration in using an optical resonant cavity for near-field heating is in coupling the optical power into the resonant cavity 12. A known technique in optical communication in coupling power into a cylindrical optical cavity is by way of evanescent-wave coupling from an integrated waveguide. As an example of this, R. W. Boyd et al., in Journal of Modern Optics, 2003, Vol. 50, No. 15-17, 2543-2550, “Nanofabrication of optical structures and devices for photonics and biophotonics” teaches a system consisting of a waveguide coupled to a resonant whispering gallery mode (WGM) cavity. The technique is schematically represented in FIGS. 2A and 2B where a tapered planar waveguide 40 is placed near a circular disk microcavity 44. The coupling can be effected if the waveguide 40 is placed with a gap 48 that is a fraction of a wavelength from the cavity. In this device the cavity and waveguide are comprised of a relatively high index of refraction material 50 such as GaAs that is surrounded by a relatively low index of refraction material, in this case air. The upper and lower surfaces of the waveguide and cavity are likewise bordered by layers 52 of relatively low index of refraction material, such as AlxGa1-xAs, where x equals 0.4.
Much of the difficulty in applying near field optical devices for TAR lies in their incompatibility with the space-limited mechanical structure of the write poles within a magnetic head, the difficulty in bringing photons to such devices, and meeting the requirements for producing a near field high intensity optical beam that is within about 10 nm from the bit area that is being written. The heated spot is preferably at or a short distance uptrack of the write pole.